互惠专一传粉榕小蜂欺骗性演化的机制

The types of species interactions mainly include neutralism, mutualism, competition, predation, inhibition and commensalism. Mutualism favours higher host specificity, and could extremely co-evolve to be obligate mutualism. However, the cases shifting from mutualism to antagonism are rare in nature. The fig-fig pollinator interaction is a classic obligate mutualism. Figs (Ficus spp.) depend on wasps for transmission of their pollen and, therefore, viable seed production, whereas fig wasps (Agaonidae) depend on fig inflorescences for the completion of their life cycle. Recent molecular work suggests that this mutualistic relationship may date back about 75 million years. There are 368 described species of fig-pollinating wasps in the world, which exists a case of reversal to ovule parasitism from active pollination in the sister taxa of fig-pollinating wasps. It is known that preventing cheating is an important mechanism to stabilize the mutualism. In recent years we have researched the cheating evolution on the ecological scale, and must add molecular technique to deeply study it now. Therefore, we will use the methods of molecular ecology to study the mechanism of cheating evolution in this project. Using microsatellite markers and DNA sequencing technology, we will research genetic polymorphism and gene flow of the cheater (Eupristina sp.), the pollinator (Eupristina altissima) and the host Ficus (Ficus altissima) so that we can discover the population genetic structure per species and know the factors driving the evolution of population genetic structure. Through co-phylogeographic studies of interacting fig and fig wasp pairs and DNA association study, haplotypes will be constructed phylogenetic relationships, combining with geological and climatic data, we can trace the co-evolutionary history, migratory route, geographical distribution pattern, possible glacial refugium, as well as the factors influencing them. Finally, we will systematically and comprehensively discover the mechanism of cheating evolution in obligate mutualism. The results may improve mutualistic symbiosis theory and provide a model for researching the cheating evolution in cooperative inter-specific interaction.

在种间相互作用中，互惠共生是一种极端的演化方式，而且互惠逆转演化的现象更为罕见。在高度互惠专一的榕-蜂共生系统中，姐妹分类单元上有一种传粉榕小蜂演化为不传粉的欺骗者，而防止欺骗是维持互惠共生系统稳定的重要机制。在前期宏观生态学研究的基础上，本项目深入从微观分子生态学角度对这例传粉榕小蜂欺骗性的演化机制进行研究。通过微卫星分子标记和DNA测序技术，研究欺骗者、传粉者和寄主高榕的种群遗传分化、多样性和基因流等，揭示它们的种群遗传结构及影响其遗传结构的主要进化因素。结合谱系地理学研究方法，构建各物种单倍型间的谱系关系，并对欺骗者、传粉者和寄主高榕的DNA序列进行联合分析，结合地史资料，追溯它们协同进化历程、迁移路线、地理分布格局形成的历史原因及可能的冰期避难所。最终,系统、全面地揭示高度互惠专一的传粉榕小蜂如何演化出欺骗性的机制，可完善互惠共生理论，并为研究合作型物种间欺骗性的演化提供模版。

在高度互惠专一的榕-蜂互惠系统中，防止欺骗是维持互惠共生系统稳定的重要机制。然而在隶属于环纹榕亚组(Conosycea)的高榕(F. altissima)和小叶榕(F. microcarpa)隐头花序中,项目组在西双版纳热带地区发现了两种Eupristina属小蜂罕见地从传粉者演化为欺骗者。本研究已采集到分布于中南半岛的25个高榕种群177个个体，选择cpDNA psbA-trnH分子标记，结果显示高山榕遗传多样性较低，只得到3个psbA-trnH单倍型，种群间遗传分化明显，基因流水平低，有97.8%的遗传变异主要来自于种群间，不存在明显的亲缘地理结构，可能经历了缓慢的种群扩张。进一步采用微卫星分子标记，使用16对SSR引物，结果显示：种群内的遗传变异具有较高比例。其种群具有四种不同的遗传组分，种群内的遗传变异贡献最大为82.33%，而组间和组内种群间分别只占到了10.07%和7.59%。部分种群经历了近期瓶颈效应。总之，对于高榕nSSR标记表现出更高的遗传多样性，而cpDNA具有更大的遗传分化。说明高榕种子和花粉具有不同的基因扩散模式，花粉流对该种群遗传多样性贡献更大。对于共存的传粉榕小蜂和欺骗性小蜂，分别测序了线粒体COI、Cytb和核基因18S、28S，并用最新的二代超级保守元件系统基因组UCEs测序Eupristina属小蜂，证实共生于高榕和小叶榕的传粉者和欺骗者均呈姐妹种关系。关于高榕传粉者和欺骗者的亲缘地理学测序、数据分析还在进行中，最终结合寄主榕树开展协同亲缘地理学分析，可以为互惠专一的传粉榕小蜂演化出欺骗性提供分子证据。本项目也从生态学、行为学、物候学等角度研究传粉榕小蜂及欺骗性小蜂共存在一种寄主榕树上的机制。发现寄主高榕主导着榕–蜂繁殖演化。此外，传粉者和欺骗者的雌蜂平均孕卵量没有差异。存在种内竞争时两种小蜂的产卵量均减少，有种间竞争时，欺骗性小蜂产卵更成功。而在比较高榕分布中心和边缘区的繁殖效率时，中心区欺骗性小蜂少，繁殖成功率高。本项目也拓展研究了部分榕树及榕小蜂共存的机制。目前已发表标注本项目资助的文章13篇（6篇SCI、其中有3篇Q1文章）、专著1部，获得学术奖励3项，培养研究生5人。